Compact discs for storing a wide variety of data types (e.g., audio, text, images, etc.) are well known in the art. As used herein, the terms "compact disc" and "disc" are defined to include all such devices, including but not limited to, CD-ROM, WORM, DVD (digital versatile disc), DVD-RAM, CD-WO (CD-Write Once), CD-R (CD-Recordable) etc. Methods and apparatus for efficient access to data stored on these discs are a significant factor for consideration in view of the proliferation of use of these discs for data storage in association with computerized information that must be readily available.
A system planner must address disc loading and retrieval, the provision of drive, data access, support and control components, and the interconnection of all elements for efficient operation. In a conventional system, a control unit operates to determine the location of a desired data block stored on a given disc. The control unit sends a control signal to the drive unit to access the desired data block. In order to facilitate data access, the drive unit initially operates to rotate the disc, which may be initially at rest, at a specified frequency. The drive unit then moves an optics set along the surface of the disc and positions the optics set adjacent the lead-in for the particular data block to be read.
It is common for Table-of-Contents (TOC) data, together with data desired to be reproduced, to be stored on a disc. The disc system may make use of Table-of-Contents (TOC) data stored on a disc to facilitate generating the control signal. The control unit directs the drive unit, via the control signal, to position the optics set to selectively access desired data. The TOC information contains various inherent data relative to the information recorded on the disc, such as the number of data blocks recorded on the disc, and the relative size of each data block, the positional address of each data block, for example. Thus, TOC data is useful for facilitating efficient access to desired data blocks. Otherwise, for example, in order for the controller to determine where data blocks are located, the entire disc would have to be read.
The time associated with causing the disc to begin rotating until the specified frequency of rotation is achieved (i.e., spin-up time) substantially slows access to the desired data. In addition, the time associated with properly positioning the optics set adjacent desired data on the disc consequently slows access to the desired data.
Furthermore, a sequence of data blocks may be stored at various portions of a disc. Thus, a desired data block may be located at one portion of the disc, the subsequently desired data block may be located at another portion, and so forth. Thus, the cumulative time associated for properly positioning and repositioning the optics set adjacent desired data blocks substantially slows data access.
A disc system may be configured with multiple disc drives. In such a configuration, a control unit operates to determine the location of a desired data block stored on a given disc, and thereby determines the location of the given disc and the associated disc drive unit. The control unit sends a control signal to the specified drive unit to access the desired data block. The drive unit must initially cause the disc to begin rotating to facilitate data access (where the disc is not already rotating). The drive unit then moves the optics sets along the surface of the disc and positions them adjacent the lead-in for the data block to be read.
The time associated with causing the disc to begin rotating until the specified frequency of rotation is achieved and the time associated with properly positioning the disc drive optics set, consequently slows access to the desired data. Furthermore, where the subsequently desired data is contained on another disc, and therefore at another disc drive, the time associated with spin-up and optics set positioning to facilitate the reading of the subsequent data block, further adds to the data access time lag.
Presently, discs may be produced with data stored on both sides (i.e., two-sided discs). Conventional systems, however, are not equipped to access data from both sides of a disc, without having to turn over the disc. The requirement of having to turn over a disc, necessarily involves a device to turn over the disc or the user must manually intervene. In either event, there is a substantial time lag involved.
It is therefore evident that there exists a need in the art for a disc optics system which mitigates time lags associated with recognizing and accessing data stored on discs.